Direct in-situ observation of hydrogen-enhanced diffusive deformation at the crack tip in pure aluminium.

Reliable service of lightweight high-strength alloys in energy efficient structures is challenged by hydrogen embrittlement. Extensive research efforts have been devoted to uncovering the mechanism underlying the phenomenon for decades, primarily focusing on the interaction between hydrogen and dislocations. Nonetheless, the explicit role of hydrogen in the direct vicinity of a propagating crack tip, wherein the deformation carriers nucleate, remains obscure. Here, by using in-situ high-resolution environmental transmission electron microscopy, we directly capture hydrogen-enhanced diffusive deformation-mediated crack propagation in pure Al, manifesting vacancy formation and atomic diffusion on the crack tip surface. This is corroborated by density functional theory calculations revealing considerably weakened Al-Al bonds at the adsorption sites and lowered energy barriers for vacancy formation and adatom diffusion. This diffusive cracking pathway is applicable under high hydrogen fugacity and low strain rates, which imposes evident effects on bulk materials. The findings point to restraining atomic diffusion to alleviate the hydrogen-assisted cracking in advanced aluminum alloys.